The extrusion of aluminum is usually preceded by heating a piece, referred to as a "billet", to the point that it is sufficiently plastic to be squeezed through an extrusion die. The die will have an opening defining the cross-section of the extruded piece. These billets are cut from elongated cast "logs", which are cylindrical, and commonly from six to ten inches in diameter. These are perhaps initially twenty feet long. One approach to supplying the heated billets is to heat the entire log, and cut off the billets as needed as they emerge from a furnace. Furnaces and cut-off equipment for performing this function have been available for some time. The long "tunnel" furnaces vary somewhat in detailed design, but usually have a conveyor system carrying the logs into one end and through the furnace, and out to the station where the hot billet is sheared from the log. A set of gas burners is usually the heat source, and these are mounted near the discharge end of the furnace. The products of combustion move toward the entrance end, causing this section of the furnace to form a pre-heat zone.
The billet supplied to the extruder does not need to be in one piece, since it will be merely a non-defined mass of plastic material in the extrusion process. The length of the billet is determined by the quantity of material required for the particular extrusion operation, and the diameter of the log. A particular extrusion operation may require billets twenty four inches long at the available log diameter. This length can be made up of one segment fourteen inches long, and another ten inches long. When the trailing end of the log approaches the shearing station, either an operator or automatic equipment must cause the system to make up segments of the required length, making allowance for the fact that the segments have a minimum length determined by the characteristics of the conveying and cut-off systems.
The standard approach to controlling the cut-off system is via a control of the incremental conveyor, or "pusher", that moves the logs through the furnace. This, in turn, has been through sensing equipment located at the entrance end of the furnace, and which records the position of the beginning of the log and of each incremental movement through the furnace. If the log were of a constant length (minus the length of pieces cut off), this control system would be adequate. However, there is a significant temperature gradient along the furnace, and the logs may remain in one section of the furnace for varying periods of time. They may remain at the hotter outlet end for an extended period, for example, as the needs of the extruders supplied by the furnace vary for one reason or another. The uncut length of a log may easily vary a couple of inches during its passage through the furnace, which would be enough to throw off the billet lengths selected for best extrusion operation. The problem is inherent in a system controlled by sensing equipment at the entrance end of the furnace.